During the distillation of heavy oils to remove valuable lighter distillates, some of the lightest constituents are removed in a fractionation vessel. For example, in a delayed coker operation of a petroleum refinery, heavy hydrocarbon (oil) is heated to about 900° F.—about 1000° F. (about 482° C. to about 538° C.) in large fired heaters and transferred to cylindrical vessels known as coke drums which can be as large as about 30 feet (about 9.1 meters) in diameter and about 140 feet (about 42.7 meters) in height. The heated oil releases its hydrocarbon vapors for processing into useful products, leaving behind solid petroleum coke which may accumulate in the drum and may reduce the efficacy of the drum for further hydrocarbon processing. The accumulated coke may be broken up and removed from the drum in the decoking cycle of the coker operation in order to prepare the coke drum for further hydrocarbon processing. Decoking may be accomplished, for example, by using high-pressure water directed through nozzles of a decoking (or coke cutting) tool.
Since flows of about 1000 gallons per minute (gpm) (about 3.79 cubic meters per minute) at about 3000 to about 6000 pounds per square inch (psi) (about 20, 684 kPa to about 41,368 kPa) can be used for such operations, it is neither practical nor desirable to open drilling and cutting nozzles at the same time. Thus diverter valves may direct the flow to the selected nozzles as required for the decoking operation. There are two commonly used diverter valve designs, both of which are complex, require numerous components, and require a very high level of precision in their manufacture in order to function. One such valve is a reciprocatable sleeve type valve having radial ports which selectively align with corresponding ports in the valve body to direct flow to either the drilling or cutting nozzles. The other is a rotatable sleeve, again having ports for selective alignment with corresponding ports of the valve body.
Many decoking tools have downward-oriented drilling or boring nozzles and sideward-oriented cutting nozzles. Decoking can be accomplished using the nozzles in two phases. First, a pilot hole, about 3 feet (about 0.9 meters) to about 4 feet (about 1.2 meters) in diameter, is cut, or drilled, downward from the top of the drum through the coke bed using the boring nozzles of the decoking tool. Then, the decoking tool is raised to the top of the vessel where either the whole tool or the cutting mode is engaged to use the cutting nozzles, and the tool, rotated and moved vertically downward in the pilot hole, cuts the balance of the coke and flushes it out the open bottom of the drum. In some aggressive operations, to reduce decoking time, the tool is changed to the cutting nozzles at the bottom of the drum, and the tool, rotated and moved vertically upward in the pilot hole, cuts the balance of the coke and flushes it out the open bottom of the drum. In this way, the raising step is skipped.
Removal of the tool from the drum to either change it out or to change its cutting mode is a cumbersome and time-consuming operation which, considering the cost and limited number of coke vessels, can significantly impact the production capacity of a refinery. Thus, there has been a continuing interest in combination decoking tools which are capable of remotely activated cutting mode shifting. For a long time, all attempts at providing such tools have failed because of mechanical jamming of mode shifting mechanisms caused by suspended coke debris in the cutting fluid. The debris is the result of recycling of the cutting fluid. Since all previous designs included some form of shuttle valve driven by through-flowing cutting fluid, all were subject to jamming due to debris carried in the cutting fluid which settled or was filtered out of the fluid and gathered between sliding surfaces of valve members. Thus, the very fluid needed to operate the shifting mechanism was the ultimate cause of the failure of the mechanism. In addition, these designs accomplished cutting mode shifting by application of full cutting fluid pressure, thereby increasing friction forces and exacerbating the jamming tendency of the debris-laden shuttle devices.
To overcome difficulties associated with the shuttle-based valve designs, the assignee of the present invention developed a relatively trouble-free, manually shiftable, combination decoking tool; such device is described in U.S. Pat. No. 5,816,505, the entirety of which is incorporated herein by reference. Additionally, a remotely operated cutting mode shifting apparatus for a decoking tool was developed and was described in U.S. Pat. No. 6,644,567 which is commonly owned herewith and is incorporated herein by reference.
Even with properly-functioning decoking tools, a coke bed may collapse during the decoking operation, particularly during aggressive operation, and trap the decoking tool within the drum. Once entrapped, the decoking tool is relatively difficult to free. Decoking tool freeing operations may take between about 4 hours to about 12 hours to remove (e.g., by flooding the drum to remove coke from the top of the drum and away from the decoking tool).
Accordingly, a need exists for alternative to systems and devices for fluid decoking.